Many modifications of Cuviers great divisions have been attempted. Some naturalists, for instance, have divided off a part of the Radiates and Articulates, insisting upon some special features of structure, and mistaking these for the more important and general characteristics of their respective plans. All subsequent investigations of such would-be improvements show them to be retrograde movements, only proving more clearly that Cuvier detected in his four plans all the great structural ideas on which the vast variety of animals is founded. This result is of greater importance than may at first appear. Upon it depends the question, whether all such classifications represent merely individual impressions and opinions of men, or whether there is really something in Nature that presses upon us certain divisions among animals, certain affinities, certain limitations, founded upon essential principles of organization. Are our systems the inventions of naturalists, or only their reading of the Book of Nature? and can that book have more than one reading? If these classifications are not mere inventions, if they are not an attempt to classify for our own convenience the objects we study, then they are thoughts which, whether we detect them or not, are expressed in Nature,then Nature is the work of thought, the production of intelligence carried out according to plan, therefore premeditated,and in our study of natural objects we are approaching the thoughts of the Creator, reading His conceptions, interpreting a system that is His and not ours.
All the divergence from the simplicity and grandeur of this division of the animal kingdom arises from an inability to distinguish between a plan and the execution, of a plan. We allow the details to shut out the plan itself, which exists quite independent of special forms. I hope we shall find a meaning in all these plans that will prove them to be the parts of one great conception and the work of one Mind.
II
Proceeding upon the view that there is a close analogy between the way in which every individual student penetrates into Nature and the progress of science as a whole in the history of humanity, I continue my sketch of the successive steps that have led to our present state of knowledge. I began with Aristotle, and showed that this great philosopher, though he prepared a digest of all the knowledge belonging to his time, yet did not feel the necessity of any system or of any scientific language differing from the common mode of expression of his day. He presents his information as a man with his eyes open narrates in a familiar style what he sees. As civilization spread and science had its representatives in other countries besides Greece, it became indispensable to have a common scientific language, a technical nomenclature, combining many objects under common names, and enabling every naturalist to express the results of his observations readily and simply in a manner intelligible to all other students of Natural History.
Linnæus devised such a system, and to him we owe a most simple and comprehensive scientific mode of designating animals and plants. It may at first seem no advantage to give up the common names of the vernacular and adopt the unfamiliar ones, but a word of explanation will make the object clear. Perceiving, for instance, the close relations between certain members of the larger groups, Linnæus gave to them names that should be common to all, and which are called generic names,as we speak of Ducks, when we would designate in one word the Mallard, the Widgeon, the Canvas-Back, etc.; but to these generic names he added qualifying epithets, called specific names, to indicate the different kinds in each group. For example, the Lion, the Tiger, the Panther, the Domestic Cat constitute such a natural group, which Linnæus called Felis, Cat, indicating the whole genus; but the species he designates as Felis catus, the Domestic Cat,Felis leo, the Lion,Felis tigris, the Tiger,Felis panthera, the Panther. So he called all the Dogs Canis; but for the different kinds we have Canis familiaris, the Domestic Dog,Canis lupus, the Wolf,Canis vulpes, the Fox, etc.
In some families of the vegetable kingdom we can appreciate better the application of this nomenclature, because we have something corresponding to it in the vernacular. We have, for instance, one name for all the Oaks, but we call the different kinds Swamp Oak, Red Oak, White Oak, Chestnut Oak, etc. So Linnæus, in his botanical nomenclature, called all the Oaks by the generic name Quercus, (characterizing them by their fruit, the acorn, common to all,) and qualified them as Quercus bicolor, Quercus rubra, Quercus alba, Quercus castanea, etc., etc. His nomenclature, being so easy of application, became at once exceedingly popular and made him the great scientific legislator of his century. He insisted on Latin names, because, if every naturalist should use his own language, it must lead to great confusion, and this Latin nomenclature of double significance was adopted by all. Another advantage of this binominal Latin nomenclature consists in preventing the confusion frequently arising from the use of the same name to designate different animals in different parts of the world,as, for instance, the name of Robin, used in America to designate a bird of the Thrush family, entirely different from the Robin of the Old World,or of different names for the same animal, as Perch or Chogset or Burgall for our Cunner. Nothing is more to be deprecated than an over-appreciation of technicalities, valuing the name more highly than the thing; but some knowledge of this nomenclature is necessary to every student of Nature.
The improvements in science thus far were chiefly verbal. Cuvier now came forward and added a principle. He showed that all animals are built upon a certain number of definite plans. This momentous step, the significance of which is not yet appreciated to its full extent; for, had its importance been understood, the efforts of naturalists would have been directed toward a further illustration of the distinctive characteristics of all the plans,instead of which, the division of the animal kingdom into larger and smaller groups chiefly attracted their attention, and has been carried too far by some of them. Linnæus began with six classes, Cuvier brought them up to nineteen, and at last the animal kingdom was subdivided by subsequent investigators into twenty-eight classes. This multiplication of divisions, however, soon suggested an important question: How far are these divisions natural or inherent in the objects themselves, and not dependent on individual views?
While Linnæus pointed out classes, orders, genera, and species, other naturalists had detected other divisions among animals, called families. Lamarck, who had been a distinguished botanist before he began his study of the animal kingdom, brought to his zoölogical researches his previous methods of investigation. Families in the vegetable kingdom had long been distinguished by French botanists; and one cannot examine the groups they call by this name, without perceiving, that, though they bring them together and describe them according to other characters, they have been unconsciously led to unite them from the general similarity of their port and bearing. Take, for instance, the families of Pines, Oaks, Beeches, Maples, etc., and you feel at once, that, besides the common characters given in the technical descriptions of these trees, there is also a general resemblance among them that would naturally lead us to associate them together, even if we knew nothing of the other features of their structure. By an instinctive recognition of this family likeness between plants, botanists have been led to seek for structural characters on which to unite them, and the groups so founded generally correspond with the combinations suggested by their appearance.